You have written �
Bb5 Possibilities are;

1-Bb �5 or Bb flat 5 Same thing =Bb D E the E being that flatted fifth.
2-B b5 B D# F -- F being the flatted fifth.
3-Bb7 b5 Bb D E Ab E being the flatted fifth. Which I doubt, there must be a 7th written in the chord.

Or possibly a Bb chord with F in the bass.
Normally by looking at the chord progression you can take a reasonable guess at want they are asking for.

Hope this helps, John C.